Quasi-isotropic thermal conductivity of polymer films enhanced by binder-free boron nitride spheres

Heat dissipation has become increasingly crucial in miniaturized modern electronics. Thanks to the high in-plane thermal conductivity yet excellent electrical insulation, hexagonal boron nitride (h-BN) has been considered as an ideal filler material to enhance the thermal conductivity of polymers for efficient heat dissipation. However, the low out-of-plane thermal conductivity significantly limits their practical applications and a heat spreader that propagates heat along multiple directions is favorable. In this work, micro-sized, binder-free boron nitride spheres (BNSs) have been successfully synthesized using a two-step process of spray drying and high-temperature sintering. Consisting of randomly dispersed boron nitride nanosheets (BNNSs), the BNSs show an isotropic thermal conductivity of 34.8 W/mK. Using the BNSs as fillers, the out-of-plane thermal conductivity of poly(vinyl alcohol) (PVA) film is significantly enhanced to 8.1 W/mK, the second highest value compared with previously reported BN based PVA composite films. In the meantime, the in-plane thermal conductivity, up to 10.6 W/mK, is not sacrificed, indicating the quasi-isotropy in thermal conductivity. The significant thermal conductivity enhancement (∼3700%) of PVA is attributed to the formation of isotropic thermally conductive networks within the polymer matrix and strong interactions between BNNSs inside BNSs. This study provides a practical route to fabricate BN-enhanced composite films with isotropic thermal conductivity and promising materials that are valuable for heat dissipation in new-era advanced electronics and related applications.